Planographic printing method and apparatus



March 20, 1962 c. E. LARSEN 3,025,790

PLANOGRAPHIC PRINTING METHOD AND APPARATUS Filed April 30, 1956 INVENTOR CARL E. LARSEN United States Patent 3,025,790 PLANOGRAPHIC PRINTING METHOD AND APPARATUS Carl Ejner Larsen, Gentofte, Denmark (6 Skovagervej, Charlottenlund, Denmark) Filed Apr. 30, 1956, Ser. No. 581,588 4 Claims. (Cl. 101-1492) In planographic printing which, as is known, rests on the basic principle that a specially prepared printing plate, when kept moistened, will be capable of repelling the fat printing ink on the non-printing areas, the lithographic stone originally used as printing plate has in all essentials today been replaced by thin metal plates, preferably zinc, aluminum or stainless steel plates or plates of a suitable artificial product, such as acetate plastic, or of paper, e.g. casein coated paper. Such plates possess essential advantages over the lithographic stone, among which are the flexibility of these plates which make it possible to fix them round a printing cylinder of a rotary printing machine and also these plates offer facilities in preparing printing images by photo-technical methods.

Such printing plates are used for large offset printing machines as well as for the smaller so called offset duplicators, but experience has shown that these plates have the drawback that, for apparently unexplainable reasons, they vary considerably as to durability. Some plates have been capable of delivering a great number of copies before they became useless while in other cases the printing image on the same type of plate and under exactly the same transfer procedure disappeared entirely or Substantially so after the delivery of a small number of copies.

An object of the present invention is to remedy the above drawback. Investigation has proved that the plate surfaces of plates which have been capable of delivering only a comparatively small number of copies were worn in such a way as to indicate that the wear was not only due to the normal wear which might occur by contact with paper fluff on the offset blanket but that a kind of corrosion had also taken place.

A further object of the invention is to provide means for counteracting such corrosion which is mainly due to electrolytic effects which arise when the printing plate is considered as one pole, the dampening liquid as the electrolyte and the inking and dampening rollers as the other pole which would set up noncontrollable electrical circuits causing such corrosion.

A still further object of the invention is to provide means for electrically insulating the printing plate from the printing cylinder and the fastening members on the cylinder.

A still further object of the invention is to provide a method whereby a layer of electrically insulating material may be disposed between the printing plate and the printing cylinder and to fastening members.

An additional object of the invention is to provide the printing plate, prior to its attachment to the printing cylinder, with an electrically insulating coating on its back and such parts of the face and the rims as may come into contact with the fastening members. This insulating coat ing which must be impervious to moisture may comprise a lacquer coating, a rubber coating, a plastic coating, or a water-impervious paper coating.

Instead of the required insulation being attained by interposing insulating intermediate layers or by coating the printing plate, the said insulation may, of course, also be obtained .by the required insulating material being present on the printing machine proper, and in accordance therewith a printing machine according to the invention is characterized in that the parts of the machine "ice with which the printing plate gets into contact are electrically insulated from the other parts of the machine. This may in a simple manner be obtained by the printing cylinder being secured to a sleeve of insulating material, such as ebonite, fastened to the shaft of the cylinder.

The required insulation may also be attained by the printing cylinder of the machine and the fastening members of said cylinder either consisting of or being provided with a coating of electrically insulating material.

Between the attached printing plate and the metal parts of the machine, inclusive of the damping and inking rollers, insulated from the said plate, a certain voltage difference is prevailing, the plate being negative in relation to the other parts, and experiments have now shown that this voltage difference which is of the order of magnitude of l to 10 mv. dependent on the nature of the plate, is of great importance for a satisfactory printing. As long as this voltage difference keeps at least substantially constant, the machine will deliver satisfactory copies. If, however, the said voltage difference alters its value the printing will no longer be satisfactory but toning" or other blemishes in the copies will occur. From this it seems to appear that the electric potential of the plate must have a definite value dependent on the nature of the plate in order that the moisture applied to the plate may cause the desired repulsion of the printingv ink thereby causing the printing ink to deposit only on the areas of the plate constituting the printing image.

Consequently, during the printing it will be expedient to be able to check the potential of the printing plate, and in accordance therewith an embodiment of the printing machine according to the invention is characterized in that it is provided with an electric voltage measuring apparatus, e.g. a millivolt metre, inserted between the attached printing plate and the metal parts of the machine insulated from said plate. In doing so, it will be possible to check the potential state of the printing plate, and if during printing it should alter its value, the printer may by altering the composition of the damping liquid or renewing the ink, restore the potential to its optimum value.

Instead of altering the potential of the plate by altering the damping liquid or renewing the ink, it is, however, also possible according to the invention to alter it electrically by applying an auxiliary voltage between the attached printing plate and the parts of the machine insulated from the said plate. In conformity herewith an embodiment of the printing machine according to the invention is characterized in that between the attached printing plate and the parts of the machine insulated from the. plate an adjustable current source is inserted, which may consist of a potentiometre fed by a dry cell or an accumulator, or by an adjustable transformer with a rectifier.

For the purpose of explaining the invention in detail reference is made to the drawing, where:

FIG. 1 is a partial perspective view of a printing cylinder with the printing plate and an insulating intermediate layer in the process of being applied thereto.

FIGKZ is a similar view of a modification showing fastening members for the printing plate of another type, and

FIG. 3 is a diagrammatic end view of a printing cylinder illustrating particularly the electrical controls for the potential between the plate and cylinder.

In the various views similar reference characters indicate like parts.

Referring to FIGURE 1, a printing cylinder 1 is secured to shaft 21 and the cylinder is provided with a groove 2 extending axially. The printing plate 3 and a layer of electrically insulating material 8 are secured to the printing cylinder by means of the clamps 4 and 5 which grip the turned over ends 9 of the insulating material between which are disposed the opposite edges of the printing plate 3. The bolts 6 secure the clamps 4 and '5 together and the bolts 7 secure such clamps to the bottom of the groove 2 whereby the plate 3 will bear upon the perimeter of the printing cylinder 1. The method outlined above also insulates the plate from the fastening members 4 and 5. In this manner the printing plate will be electrically insulated from the metal parts of the printing machine.

In the modification shown in FIGURE 2, the printing plate 3 which is to be attached to the printing cylinder 1" has its back coated with a layer of insulating material 10. In this modification the fastening members consist of a row of pins 11 secured to one of the side walls of the groove or recess 2'. Another row of pins 12 is provided upon a frame member 13 disposed parallel to the other side wall of the groove 2. The ends of the frame member 13 are formed as levers 14 which are pivotally connected to pins 15 located upon the end walls of the printing cylinder. A spring 16 has one end fixed to a pin 17 in one end wall of the cylinder and its other end is secured to an arm 14. The spring urges the frame member 13 towards the bottom of the groove 2'.

The printing plate 3 is provided at two opposite edges with a row of holes 18 and 19 into which the pins 11 and 12 engage. In attaching the printing plate 3' the holes 18 are first brought into engagement with the pins 11 and then the plate is laid around the printing cylinder. The holes 19 are then brought into engagement with the pins 12 and the spring 16 will lower the member 13 down towards the bottom of the groove 2 thereby fitting the printing plate snugly around the perimeter of the cylinder 1. Also insulating material 20 is provided around the edges of the holes 18 and 19 thereby insulating the plate 3' from the pins 11 and 12.

Of course, the printing plates 3 and 3' need not to be provided with an insulating coating or be fitted together with an insulating intermediate layer if the required insulation is otherwise obtained.

This may be attained by making the printing cylinder, the pins 11 and 12 and the frame member 13 of an insulating material or at least provide these parts with a coating of such material.

This may also be attained by insulating the entire printing cylinder 1 or 1' from its shaft 21 and consequently from the other parts of the machine. An embodiment showing this solution is indicated in FIG. 2 where a sleeve 33 of an insulating material, e.g. ebonite, is secured'to the shaft 21, and the printing cylinder is then secured to said sleeve 33.

In FIG. 3 the printing cylinder 1' is provided with a printing plate 3' attached thereto, the latter being in conformity with one of the above methods electrically insulated from the printing cylinder and the fastening members. The shaft 21 of the printing cylinder is provided with a ring 22 of electrically insulating material, the said ring 22.being externally provided with a conductive contact ring 23 which by means of a lead 24 is conductively connected to the printing plate 3". If the shaft 21 is provided with an insulating sleeve 33, the ring 22 may form part of said sleeve. A terminal 25 is by a contact spring 26 sliding on the contact ring 23 conductively connected to the printing plate 3'. Another terminal 27 is connected to the metal parts of the machine, e.g. by means of a contact spring 28 sliding on the shaft 21. Between the terminals an electric voltage measuring apparatus, e.g. a millivolt metre 29, is inserted.

By this arrangement it is possible during operation of the printing machine to measure the voltage difference between the printing plate 3' and the metal parts of the machine insulated from said plate, the said metal parts including also the rollers of the inking duct and the damping duct, of which the drawings show three rollers 30.

If during operation disturbances occur in the nature of poorer printing, this will manifest itself in an alteration of the voltage difference, and the operator will consequently be aware of the cause of the unsatisfactory running. He can then take adequate steps to bring the operation of the machine into order again.

By the present invention this may be effected in a very simple manner, namely, by supplying from the outside an auxiliary voltage being capable of restoring the voltage difference to normal. To this end an adjustable current source is connected to the terminals 25 and 27. In FIG. 3 the said current source consists of a potentiometer 31 fed by a storage battery 32. However, the current source may also be made in another way; it may, e.g., consist of an adjustable transformer comprising a rectifier, since the supplied auxiliary voltage must in the nature of things be a DC. voltage.

It is thought that the invention and its advantages will be understood from the foregoing description and it is apparent that various changes may be made in the process, form, construction and arrangement of the parts without departing from the spirit and scope of the invention or sacrificing its material advantages, the form here inbefore described and illustrated in the drawings being merely a preferred embodiment thereof.

I claim:

1. A planographic printing machine comprising a printing cylinder, a shaft for said printing cylinder, a sleeve of electrically insulating material on said shaft upon which said printing cylinder is mounted, means for measuring the potential difference between said shaft and said cylinder and means for controlling said potential difference to maintain the same at a predetermined value.

2. A planographic printing machine comprising a printing cylinder, a printing plate mounted upon said cylinder, means electrically insulating said plate from said cylinder interposed between said plate and cylinder, a millivolt meter electrically connected to said plate and to said cylinder to measure the potential difference therebetween and an adjustable potential source inserted between said printing plate and the parts of the machine insulated from said plate.

3. *In a planographic printing process upon a printing machine of the kind in which the printing plate is electrically insulated from the machine frame the steps of measuring the potential difference between the printing plate and the machine frame and then controlling said potential diflierence by an auxiliary potential to maintain the same constant at the proper value for the desired printing quality.

4. In a planographic printing process upon a printing machine of the kind in which the printing plate is electrically insulated from the machine frame the steps of measuring the potential difference between the printing plate and the machine frame and then controlling said potential difference'by impressing an additional voltage from an outer voltage source across the printing plate and the machine frame to maintain said potential difference'constant at the proper value for the desired printing quality.

References Cited in the file of this patent UNITED STATES PATENTS 1,357,340 Novotny Nov. 2, 1920 1,677,655 Reed et al July 17, 1928 1,895,125 Durham Jan. 24, 1933 

